Smelting of copper oxides to produce blister copper

ABSTRACT

Blister copper is produced from copper oxides or dead-roasted copper sulfides in a cyclic smelting process using a carbon reductant. In the first stage of the cycle, carbon reductant is added to the furnace charge in stoichiometric excess to form a copper-depleted slag and a molten black copper fraction. The slag is tapped, leaving black copper in the furnace, and a second charge containing carbon is stoichiometric deficiency is then smelted to form a copper-rich slag and blister copper. Blister copper is then tapped, leaving copper-rich slag in the furnace, and the cycle is repeated.

United States Patent [191 Hunter et al.

SMELTING OF COPPER OXIDES TO PRODUCE BLISTER COPPER Assignee:

Filed:

Inventors: Willard L. Hunter; William A.

Stickney, both of Albany, Oreg.

The Unites States of America as represented by the Secretary of theInterior, Washington, DC.

Sept. 26, 1973 Appl. No.: 401,002

US. Cl 75/74, 75/72, 75/73,

Int. Cl CZZb 15/00 Field of Search 75/74, 73, 72, 63, 89

References Cited UNITED STATES PATENTS 3,725,044 4/1973 Morisaki et a175/74 OTHER PUBLICATIONS Latimer, W. et 211., Reference Book ofInorganic Chemistry, New york, 1951, pp. 254-256, (QD151L3).

Primary Examiner-Walter R. Satterfield Attorney, Agent, or FirmRoland H.Shubert; Frank A. Lukasik [57] ABSTRACT Blister copper is produced fromcopper oxides or dead-roasted copper sulfides in a cyclic smeltingprocess using a carbon reductant. In the first stage of the cycle,carbon reductant is added to the furnace charge in stoichiometric excessto form a copper-depleted slag and a molten black copper fraction. Theslag-is tapped, leaving black copper in the furnace, and a second chargecontaining carbon is stoichiometric deficiency is then smelted to form acopper-rich slag and blister copper. Blister copper is then tapped,leaving copper-rich slag in the furnace, and the cycle is repeated.

11 Claims, 3 Drawing Figures COPPER SULFIDES PCT.

PATENTED 1 IBM 3; e57. 701

SHEET 10F 2 TO STACK l4 ACID l5 PLANT ACID be FLUX I I9 1 20 8x OFF-GASROASTER f CALCINE 3 ELECTRIC if [23 FURNACE SLAG TCOPPER AIR Cu IN SLAG,

Fe IN METAL, PCT.

CARBON, PCT. OF STOICHIOMETRIC P-ATEN-TEU-BEEB'II974" 6357,70 1

'-.snm2nr2= I Cu IN SLAG, PCT.

l I I l O 0.! 0.2 0.3 0.4

Fe IN METAL, PCT.

FIG. 3.

SMELTING OF COPPER OXIDES TO PRODUCE BLISTER COPPER CROSS-REFERENCE TORELATED APPLICATION This patent application is related to patentapplication Ser. No. 401,004, filed of even date herewith, which isdirected to a process for Smelting of Nickel Oxide Ores to ProduceFerronickel by Willard L. Hunter, Danton L. Paulson and William A.Stickney. The two patent applications are commonly assigned.

BACKGROUND OF THE INVENTION The extractive metallurgy of copper sulfideconcentrates, as it is conventionally practiced today, consists of threemain steps; roasting, rcverberatory smelting and converting. Roasting isusually continued to the point whereat most of the iron sulfidecontained in the copper concentrate is converted to the oxide leaving acalcine composed of copper sulfide, iron oxide, some iron sulfide andthe gangue which was originally present in the concentrate, Depending onthe type of ore and the roasting conditions, the off-gas from roastingcontains from about 1 to 12% sulfur dioxide. In the reverberatorysmelting step, the calcine is smelted, usually with silica and limeadditions, to produce matte and a slag. The composition of the matte isessentially a mixture of copper and iron sulfides. Gases generated fromthe smelting step generally range from about 0.25 to 2.5% sulfur dioxidecontent depending on smelting conditions and sulfur concentration of thecalcine. Matte from the smelting step is treated further in a converterwhere it is blown with air to oxidize the iron sultide and to reduce thecopper sulfide to copper metal. Sulfur dioxide content of the off-gasmay range from about 3 to 12% during the converting cycle.

Copper smelters today are faced with the problem of treating theoff-gases from these three major unit operations to remove a minimum of90% of the sulfur oxides from the gas streams before venting. Theestablished method for sulfur oxide removal at this time is theproduction of sulfuric acid by the contact process. Contact acid plantsrequire a'minimum of about 3.5 to 4% sulfur dioxide content in the feedgas to economically and efficiently convert the sulfur oxides to acid.Roaster off-gases provide an appropriate feed for a sulfuric acid plantbut the amount of sulfur conventionally removed from a copper sulfideconcentrate in the roasting step is only in the range of 30 to 40%.About a quarter at most of the remaining sulfur is removed in thercverberatory furnaces but the sulfur dioxide con tent of theseoff-gases is too low to constitute an appropriate acid plant feed.Nearly half of the total sulfur contained in a typical copper sulfideconcentrate remains to be removed in the converters. Concentration ofsulfur dioxide in these off-gases varies during the converting cycle andgas processing is further complicated by the fact that the converteroperation is a batch process. However, if multiple converters are used,their operation can be phased to smooth out major variations in gas flowand sulfur dioxide content.

Because of the problems and limitations imposed by current smeltingpractice, it is difficult for the copper industry to economically meetthe imposed 90% collection standards for sulfur dioxide using presentlyavailable technology.

SUMMARY OF INVENTION Smelting of copper oxides may be accomplished in atwo-stage cyclic smelting process using a carbon reductant to produceblister copper having a purity comparable to that produced bytraditional methods and, at the same time, holding copper levels in theslag to about 1%. The first stage of the cycle is accomplished undercarbon-rich conditions to yield a copper-depleted slag while the secondstage is accomplished under carbonpoor conditions to yield a highquality, blister copper.

In a preferred embodiment, a copper sulfide concentrate is dead-roastedto yield a copper oxide calcine and a sulfur oxide-rich off-gas. Theroaster off-gas is then treated in a contact process acid plant torecover more than of the sulfur originally present in the sulfideconcentrate as sulfuric acid. Calcine is then smelted with coke in anelectric furnace to yield blister copper and a lean slag.

Hence, it is an object of our invention to produce blister copper from acopper sulfide concentrate while recoverying more than 90% of thecontained sulfur.

Another object of our invention is to produce blister copper and a leanslag from copper oxide by carbon reduction.

Yet another object of our invention is to smelt deadroasted coppercalcine in an electric furnace without significant copper loss to theslag.

DETAILED DESCRIPTION OF THE INVENTION The invention will be describedwith reference to the accompanying drawings in which:

FIG. 1 is a diagrammatic flow sheet representing a preferred embodimentof our process.

FIG. 2 is a plot representing the relationship of copper content in theslag to iron content in the metallic copper.

FIG. 3 graphically represents the relationship of copper contained inthe slag to iron contained in the cop-- per as a function of the amountof carbon in the fur nace charge. These figures will be discussed inmore detail later.

The sulfide ores of copper constitute the most important source ofcopper metal. Iron is almost invariably present in significant amount incopper sulfide concentrates either in the form of mixed copper-ironsulfides such as chalcopyrite and bornite or in the form of ironsulfides such as pyrite and marcasite which are associated with coppersulfidesin the ore body.

In conventional practice, only a portion of the sulfur contained in acopper sulfide concentrate is removed in the roasting step since thepresence of sulfur is required to form a matte during rcverberatorysmelting. It is well known that roasting can be continued to the pointwhere all of the concentrate is in the oxide form. This is commonlyreferred to as dead-roasting and may be accomplished in a fluidized'bedreactor provided that care is taken to avoid fusion within the reactionzone. It is also known that such dead-roasted calcines may be reducedwith carbon to produce a metal product known as black copper which isessentially an alloy of copper with iron and small amounts of variousother metals. Principally becuase of its high iron content, black copperis unsuitable for electrolytic refining. If a selective reduction isattempted to avoid the reduction of iron oxides to the elemental formand thus obtain a more highly pure copper metal product, then copperlosses to the slag become so high as to be unacceptable.

Yet from the standpoint of controlling sulfur oxide emissions,dead-roasting is attractive since essentially all of the sulfur isremoved in the roaster off-gas at a concentration of about 10 to 15%.Gases of such composition can be easily and economically treated by thecontact process to recover the sulfur oxides as sulfuric acid. Anotheradvantage accruing from this approach is that the composition and volumeof gases to be treated can be maintained at a relatively constant level.

We have found that dead-roasted copper calcines may be reduced in acyclic process to recover copper metal of comparable purity t theblister copper produced in conventional processes and to produce a leanslag containing no more, and in some cases considerably less, copperthan is discarded in the converter and reverberatory slags of thetraditional process.

Referring now to FIG. 1, there is shown a stylized flow sheet of ourprocess. A copper sulfide concentrate is introduced into a roaster 11where it is contacted at elevated temperatures with an oxygen-containinggas such as air 12. Roaster 11 may comprise a kiln of multiple hearthtype roasting furnace but is preferably a multistage, fluidized bedreactor. The ignition temperature for copper sulfide concentrates is onthe order of 350 to 400C but the dead roasting operation must be carriedout at temperatures high enough to decompose intermediate sulfates andoxy-sulfates formed at lower temperatures. In practice, temperatures ofabout 750 to 900C are employed. Care must be taken to avoid fusionwithin the reaction zone which occurs at temperatures as low as about900C depending upon the gangue constituents present. Air is convenientlyused to provide oxygen for the reaction but oxygen or oxygen-enrichedair may also be employed to increase the sulfur dioxide content of theroaster off-gases.

Roaster off-gases 13, containing from about 10 to 15% sulfur dioxidewhen air is used as the oxidizing medium, are employed as a feed streamto an acid plant 14 to produce a sulfuric acid by-product l5 and asulfur oxide-depleted gas stream 16 suitable for venting through astack. It is generally necessary to cool the roaster off-gases prior tointroduction into the acid plant and this may be accomplished using awaste heat boiler (not shown). Well over 90% of the sulfur contained insulfideconcentrate 10 may be recovered in the by-product sulfuric acidstream 15.

A copper calcine l7, essentially depleted in sulfur and containingcopper oxides, iron oxides, gangue constituents and minor amounts ofother metal oxides, is recovered from roaster l1. Calcine 17 is thenintroduced as a component of the charge to furnace 18 which ispreferably an electric furnace of the submerged arc type. Added to thefurnace charge is a carbon-containing reductant such as coke l9 and afluxing agent 20. Composition and amount of flux 20 added as a componentof a furnace charge is determined by the gangue composition of calcinel7 and by the concentration of impurity metal oxides, especially ironoxides, contained in the copper calcine.

Fluxing agents useful in our process include silica, lime andcombinations thereof.

As has been stated before, ours is a cyclic process; one complete cycleconsisting of two stages. In the first stage, carbon is added instoichiometric excess and, upon heating to a molten state, there isproduced a sop slag fraction essentially depleted of copper and a lowerblack copper fraction containing substantial amounts of iron dissolvedin molten copper. At this point, the slag 21 is tapped while the blackcopper is retained in the furnace. A second charge is then added to thefurnace. The second charge consists of copper calcine, fluxing agentsand carbon in an amount substantially less than stoichiometric. Thiscarbon deficiency produces in the second stage of the cycle a slag whichis quite rich in copper and molten copper fraction which is low in ironand of purity comparable to blister copper. At this point, the moltenblister copper 22 is tapped from the furnace but the molten copper-richslag is retained. This sequence ofoperations completes one cycle whichthen may be repeated indefinitely.

During the reduction, there is produced a relatively small amount offurnace off-gas 23 which contains those fixed gases present in thefurnace charge and the gaseous produces of reduction which are primarilycarbon dioxide, carbon monoxide and small amounts of sulfur oxides.These furnace off-gases are desirably treated before being vented. Thismay conveniently be accomplished by recycling the furnace off-gas to theroaster wherein carbon monoxide will burn to the dioxide. Any sulfuroxides present in the furnace offgas will be removed in the acid plant14. Since the volume-of furnace off-gas is very small compared to thevolume of roaster off-gas, recycle of the furnace off-gas in the mannerdescribed does not materially affect operation of the roaster.

Slag fraction 21 will typically contain about 171 copper. Residualcopper contained in the slag is mostly in the form of small metallicprills physically suspended in the slag. Copper content of the slag isdependent to a large extent upon smelting conditions and upon slagcharacteristics; especially upon slag fluidity. A substantial part ofthe residual copper contained in the slag may be recovered bycomminuting the slag to a size range whereat there is substantialphysical liberation of the copper prills and thereafter subjecting theground slag to a flotation treatment. The flotation is of conventionaltype and may readily be accomplished using xanthates as promoters andflotation conditions typical of those used in the flotationconcentration of copper sulfide ores. A rougher flotation concentrateconsisting mostly of metallic copper and copper sulfides, may berecycled to the smelting furnace for recovery of the contained copper.

It is preferred that the furnace charge be added in a premixed state;that is as a mixture of calcine, flux and carbon reductants. In thismode, it is necessary to prepare furnace charges 'of two compositions;one a high carbon charge fed to the first stage of the cycle andtheother a low carbon charge fed to the second stage of the cycle. Thecarbon reductant may be in the form of shell carbon, petroleum coke andlike materials.

In the first stage of the cycle, carbon is present in stoichiometricexcess while in the second stage of the cycle it is present instroichiometric deficiency. Stoichiometry is based upon the reduction ofcopper compounds present in the calcine to elemental copper with carbonbeing converted to carbon monoxide. In the first stage, copper contentof the slag is low and iron content of the copper is high. The reversesituation occurs in the second stage. This relationship betweenconcentration of copper in the slag and iron concentration in the liquidcopper metal is plotted as FIG. 2. As may be seen from that figure, anappreciable concentration of copper oxide in the slag resulted in a lowiron content, usually less than 0.05%, in the metal product. At copperconcentrations of about 4% and lower in the slag amounts of iron in themetal increases rapidly.

FIG. 3 represents similar data to that of FIG. 2 but is plotted as afunction of the amount of carbon included in the furnace charge. On thebasis of operating data such as is presented in FIG. 3, we prefer thatthe furnace charge to the first stage of the cycle contain more than120% of the stoichiometric carbon requirements and the furnace charge tothe second stage of the cycle contain less than 80% of thatstoichiometrically required. In our most preferred mode of operation,carbon in the first stage furnace charge is more than about 160% ofstoichiometric while carbon in the second stage furnace charge is lessthan about 40% of stoichiometric.

As may now be appreciated, our process has a number of substantialadvantages compared to these of the prior art. First, essentially all ofthe sulfur contained in the concentrate is removed during one unitoperation at steady rates. Second, the concentration of sulfur dioxidein waste gases may be maintained at substantially constant levels thusallowing more efficient and trouble free operation ofthe acid plant.Third, the total volume ofwaste gases produced by our process is lessthan half produced by the typical roaster-reverberatory furnaceconverter process now commonly employed. Finally, our process requiresbut one slagging step instead of two as is conventional thus decreasingthe amount of slag produced as a waste product and in some instancesdecreasing total copper loss to the slag.

The following example serves to more fully illustrate our invention.

EXAMPLE A series of cyclic tests were conducted in an 800 Kva electricarc furnace at feed rates of approximately 2,000 pounds per hour. Eitherslag or metal was tapped hourly. The copper calcine fed to the processhad the following composition in weight percent: Cu, 38.0; Fe, 29.0; SiO3.1; MgO, 0.03; A1 0 0.9; CaO, 0.3; Pb, 0.09 and Zn, 2.1. In addition,the calcine contained 0.04 02 Au and 37.6 02 Ag per ton.

Carbon addition to the first stage of the cycle was 170% ofstoichiometric while carbon addition to the second stage was 30% ofstoichiometric. sufficient silica and dead-burned lime were added asfluxes to produce a slag-to-metal ratio of 0.7. Slag tapped from thefirst stage of the cycle contained 1.1 to 1.5% copper. Iron content ofthe molten copper tapped from the second stage of the cycle wasconsistently below 0.05%.

Slag tapped from the smelting process was crushed and ground to anominal size range of 200 mesh. The ground slag was slurried in water toform a pulp and was then subjected to a conventional froth flotationseparation using a xanthate promoter. Flotation conditions were typicalof those uskd to concentrate copper sulfide ores.

A rougher flotation concentrate, comprising tiny prills of metalliccopper and copper sulfides was recycled to the smelting furnace. Theflotation step reduced the copper content of the slag from a level of1.1 1.5% as tapped to a level of 0.4 to 0.7% as cleaned. Thus, totalcopper losses to the slag amounted to 0.3 to 0.5% of the copper in thecalcine fed to the smelting furnace.

We claim: 1. A process for reducing copper oxides to form blister copperwhich comprises:

heating in a furnace a first charge comprising copper oxides and acarbon reductant in an amount sufficient to provide at least of thecarbon stoichiometrically required to reduce the copper oxides containedin said first charge to copper metal to form a liquid slag depleted incopper and a liquid black copper phase; tapping from the furnace asubstantial portion of the copper depleted slag; adding to the furnace,still containing the liquid black copper phase, a second chargecomprising copper oxides and a'carbon reductant in an amount less than80% ofthe carbon stoichiometrically required to reduce the copper oxidecontained in said second charge to copper metal and heating the chargetapping from the furnace a substantial portion of the blister copper.

2. The process ofclaim 1 wherein a slag-forming flux is added to boththe first and second furnace charges.

3. The process ofclaim 2 wherein said copper oxides are a calcineproduced by the dead roasting ofa copper sulfide concentrate.

4. The process of claim 3 wherein said roasting is carried out attemperatures above about 750C but below the fusion point of theconcentrate.

5. The process of claim 4 wherein off-gases from the roasting step aretreated in a contact process sulfuric acid plant to produce a sulfuricacid product and a residual gas depleted in sulfur oxides. v

6. The process ofclaim 5 wherein the concentration of sulfur oxides inoff-gases from the roasting step is maintained at a level about about10%.

7. The process ofclaim 6 wherein said carbon reductant is coke.

8. The process of claim 6 wherein off-gases from the furnace arerecycled to the roasting step.

9. The process of claim 7 wherein the first charge contains more thanabout of the carbon stoichiometrically required to reduce the copperoxides to copper metal and wherein the second charge contains less thanabout 40% of the carbon stoichiometrically required to reduce the copperoxides to copper metal.

10. The process of claim 2 wherein the copperdepleted slag iscomminuted'and thereafter subjected to a flotation separation to recovera concentrate com prising metallic copper.

11. The process ofclaim 10 wherein the concentrate is recycled as a feedto the smelting furnace.

1. A PRCESS FOR REDUCING COPPER OXIDES TO FORM BLISTER COPPER WHICHCOMPRISES: HEATING IN A FURNACE A FIRST CHARGE COMPRISING COPPER OXIDESAND A CARBON REDUCTANT IN AN AMOUNT SUFFICIENT TO PROVIDE AT LEAST 120%OF THE CARBON STOICHIOMETRICALLY REQUIRED TO REDUCE THE COPPER OXIDESCONTAINED IN SAID FIRST CHARGE TO COPPER METAL TO FORM A LIQUID SLAGDEPLETED IN COPPER AND A LIQUID BLACK COOPER PHASE; TAPPING FROM THEFURNACE A SUBSTNTIAL PORTION OF THE COPPER DEPLETED SLAG; ADDING TO THEFURNACE, STILL CONTAINING THE LIQUID BLACK COPPER PHASE, A SECOND CHARGECOMPRISING COPPER OXIDES AND A CARBON REDUCTANT IN AN AMOUNT LESS THAN80% OF THE CARBON STOICHIOMETRICALLY REQUIRED TO REDUCE THE COPPER OXIDECONTAINED IN SAID SECOND CHARGE TO COPPER METAL AND HEATING THE CHARGETO FORM A LIQUID SLAG RICH IN COPPER AND A BLISTER COPPER PHASE, ANDTAPPING FROM THE FURNACE A SUBSTANTIAL PORTION OF THE BLISTER COPPER. 2.The process of claim 1 wherein a slag-forming flux is added to both thefirst and second furnace charges.
 3. The process of claim 2 wherein saidcopper oxides are a calcine produced by the dead roasting of a coppersulfide concentrate.
 4. The process of claim 3 wherein said roasting iscarried out at temperatures above about 750*C but below the fusion pointof the concentrate.
 5. The process of claim 4 wherein off-gases from theroasting step are treated in a contact process sulfuric acid plant toproduce a sulfuric acid product and a residual gas depleted in sulfuroxides.
 6. The process of claim 5 wherein the concentration of sulfuroxides in off-gases from the roasting step is maintained at a levelabout about 10%.
 7. The process of claim 6 wherein said carbon reductantis coke.
 8. The process of claim 6 wherein off-gases from the furnaceare recycled to the roasting step.
 9. The process of claim 7 wherein thefirst charge contains more than about 160% of the carbonstoichiometrically required to reduce the copper oxides to copper metaland wherein the second charge contains less than about 40% of the carbonstoichiometrically required to reduce the copper oxides to copper metal.10. The process of claim 2 wherein the copper-depleted slag iscomminuted and thereafter subjected to a flotation separation to recovera concentrate comprising metallic copper.
 11. The process of claim 10wherein the concentrate is recycled as a feed to the smelting furnace.